It is well known that when polyolefin such as polyethylene or polypropylene is incorporated with inorganic fillers such as calcium carbonate, barium sulfate and talc, the thus incorporated polyolefin is improved in rigidity. On one hand, 4-methyl-1-pentene polymer which is a kind of polyolefin is excellent in heat resistance, chemical resistance, transparency, mold release characteristics, etc., as compared with other polyolefins, but from the viewpoint of molecular structure there was such a problem that because of its low melt tension, the polymer is difficult to thermoform. Further, there was another problem that the incorporation into 4-methyl-1-pentene polymer of such inorganic fillers as mentioned above results in improved rigidity at high temperatures, but the toughness at low temperatures of the polymer thus incorporated comes to decrease thereby.